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STA SPM Single Point Mooring Analysis
Revision 1
Page i
STA SPM
A PROGRAM FOR STATIC ANALYSIS OF
SINGLE POINT MOORED VESSELS
USER MANUAL AND
TECHNICAL DOCUMENTATION
Revision 1a
Manual Prepared by
STEWART TECHNOLOGY ASSOCIATES
728 Bering Drive, Unit M
Houston, TX 77057
Tel: (713) 789-8341
Fax: (713) 789-0314
www.stewart-usa.com
E-Mail: [email protected]
July 2, 2007
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007
STA SPM Single Point Mooring Analysis
Revision 1
Page ii
EXTRACTS FROM LICENSE AGREEMENT
LIMITATION OF USE
This License is granted to the USER for an indefinite period. The USER agrees that no individual,
outside consultant, government organization, or any person who is not on permanent staff with the
USER or under direct in-house control of USER shall have access to the PROGRAM or shall use
the PROGRAM for any purpose at any time. The use of the PROGRAM is not limited to a single
machine, and the USER may make copies of PROGRAM and run it on several machines
simultaneously. The USER agrees to make any reasonable effort to assure that the PROGRAM
file or disk is not copied without authorization by OWNER, and that all users in USER's
organization are familiar with these Limitations of Use. The USER agrees not to modify, copy,
sell, lease, rent, give free of charge, or otherwise distribute or alter the PROGRAM or any part
thereof to any individual, government agency, or organization outside of the USER organization.
COPYRIGHTS
All copyrights to the PROGRAM are reserved by OWNER. All versions of the PROGRAM are
copyrighted by OWNER worldwide, beginning with 1991. The following is a trademark of
OWNER: STA SPM. The USER shall clearly and distinctly indicate the copyright in all published
and public references to the PROGRAM.
WARRANTY
While the OWNER has carefully developed the software and the software has been tested for
accuracy and proper functioning, nevertheless the OWNER cannot guarantee its accuracy and
correctness. If the software fails to perform correctly as a result of errors or omissions by the
OWNER or its staff, the OWNER will at its discretion rectify those errors and omissions free of all
charges to the USER. This shall be the limit of the OWNER's liability in this respect. OWNER
warrants that it has the right to grant this license. The PROGRAM and its documentation is sold
"as is," and the USER assumes the entire risk as to quality and performance.
HOLD HARMLESS
The OWNER shall not be liable to the USER or any other party for any design, performance or
other fault or inadequacy of the PROGRAM or its manual, or for any direct or implied damages of
any kind arising out of or in any way related to or connected with any use of the PROGRAM.
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007
STA SPM Single Point Mooring Analysis
Revision 1
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CONTENTS
Section and Subject
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Page No.
INTRODUCTION ........................................................................................................................... 1
APPLICATION............................................................................................................................... 3
THEORY ....................................................................................................................................... 3
RESULTS ...................................................................................................................................... 3
LOADING AND RUNNING THE PROGRAM ................................................................................ 4
ADDITIONAL FEATURES............................................................................................................. 7
SOFTWARE REQUIREMENTS .................................................................................................... 9
HARDWARE REQUIREMENTS ................................................................................................... 9
REFERENCES .............................................................................................................................. 9
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007
STA SPM Single Point Mooring Analysis
1.0
Revision 1
Page 1
INTRODUCTION
The calculation of the equilibrium position of a tanker, or any other ship, at a single
point mooring is complicated when the wind, wave, and current directions are not the
same. In many cases, the equilibrium position of the vessel is such that it is completely
non-intuitive. This is especially the case in relatively shallow water where the current
forces of the vessel increase significantly, primarily in the beam direction, when the
water depth to draft ratio becomes less than 1.2. Stewart Technology Associates (STA)
has introduced a new tool for the evaluation of the static equilibrium position of vessels
at single point moorings.
STA SPM is a program for the static analysis of the equilibrium position and mooring
forces on single point moored vessels. It has been developed specifically for the
analysis of single point moored tankers, either with bow turrets or with bow hawsers. A
second tandem offloading tanker together with astern thrust or with an attendant tug
can also be simulated. The program is extremely fast and efficient. A display of the
moored vessels is shown in plan view together with the directions of the wind, wave,
and current conditions.
The program, in its simplest form, computes the equilibrium position of a vessel moored
by a single point at the bow. Wind speed and direction, current speed and direction, as
well as wave height and direction are specified by the user. All of these environmental
effects can come from different directions. The program is sensitive to the water depth
to draft ratio of the moored vessel. In a few seconds the equilibrium position is found
together with all steady forces acting and the tension in the mooring hawser (or
magnitude and direction of force in a bow turret).
In conditions where a tanker is moored by the bow and has a second tanker involved
with product offtake, it is simple to use STA SPM in order to calculate the equilibrium
position of both vessels under the action of wind, waves, and current. The addition of a
tug at the stern of the offtake vessel can often improve the equilibrium stability and
reduce the total hawser tensions, or tensions applied to the mooring system. This
condition, too, is simple to analyze with STA SPM.
The results of an analysis are largely contained on a single page (see Figure 1 on the
next page). When the analysis begins the user is offered a series of dialog boxes as
shown in Figures 2 through 9 (see section 5). From these dialog boxes the user is
quickly able to select whether to analyze a single vessel, a vessel with a tandem vessel,
or a vessel with tandem vessel and tug attached. A dialog box prompts the user to
input the nominal size for the vessel attached to the mooring and offers a drop down list
of typical tanker sizes. When the first vessel has been selected, another dialog box
pops up offering the user the possibility of changing the principal dimensions of the
tanker selected in the previous step. This is repeated for the second (tandem) vessel.
Through additional dialog boxes the user selects the environmental conditions to be
analyzed, including the water depth at the site. If required, additional dialog boxes
prompt the user to input data for the aft thrust available from either the vessel main
screws or from and attendant tug.
© Stewart Technology Associates
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July 2, 2007
STA SPM Single Point Mooring Analysis
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Page 2
Figure 1. An example of results from STA SPM
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007
STA SPM Single Point Mooring Analysis
2.0
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APPLICATION
The program is intended for a screening tool to determine the levels of force applied to
hawsers and mooring systems when vessels are to be moored at single point moorings.
The user can quickly evaluate the effect of different directions of wind, waves, and
current and can determine the static, or mean, forces applied to the mooring system
under various conditions. From these results the general size of a hawser required for
a single point mooring can be determined; similarly the general size for mooring chains
and anchors can be determined.
3.0
THEORY
The program uses a combination of coefficients and calculation techniques published
by the OCIMF (Oil Companies International Marine Forum) and the US Navy Design
manuals in order to compute wind and current forces on the moored vessels. In order to
compute wave drift forces, the program presently adopts the methodology and practical
data contained in API RP 2SK, with some modifications. A bisection iteration scheme
is employed to determine vessel equilibrium headings. The program is an advanced
Microsoft Excel application and uses the VISUAL BASIC programming language. All
this is completely transparent to the user.
Future enhancements are planned to the program which will include a user defined
wave spectrum. The user will be able to select from various standard wave spectra or
to input their own wave spectrum. The program will then compute not only the mean
drift force components but also the dynamic ship motions caused by slow drift
oscillations. Additional input required from the user will include the stiffness of the
mooring system to which the first vessel is attached.
In a future release of the program, there will also be automatic computation of the
vessel wave frequency motion (heave, surge, pitch, sway, roll, and yaw). In order to
compute these motions the user may either specify the RAOs (Response Amplitude
Operators) directly or simply specify a few terms, including vessel VCG and the
program will approximate the RAOs.
Output from the dynamic version of the program will include all necessary terms to be
applied to the mooring system in order to determine peak mooring tensions either in a
dynamic simulation or in a quasi static sense.
4.0
RESULTS
Results from the program are extremely simple to interpret. A picture of the moored
vessel(s) is shown in plan view and a summary of the applied environmental conditions,
vessel sizes, and resulting loads, including hawser tension(s) are given on a single
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007
STA SPM Single Point Mooring Analysis
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page. It should be noted that all results are static results coming from the combination
of the mean components of force resulting from wind, waves, and current.
5.0
LOADING AND RUNNING THE PROGRAM
The program is written in the form of a Microsoft Excel spreadsheet. To run the
program simply double click on the STA SPM icon from the program manager. When
the file has loaded, you will be prompted to enter data in a series of dialog boxes.
Figure 2. Dialog box for selecting the basic
configuration.
The first dialog box, “Basic
Configuration”, is shown in
Figure 2. This dialog box
allows the user to enter the
preliminary
information
about the system to be
analyzed.
Through this
dialog box, the user lets the
program know how many
vessels are attached to the
mooring, the type of angular
input for the weather
direction, and whether or not
there is any aft thrust
applied to the vessel. Note
that aft thrust is applied to
only the second vessel if
there are two vessels in the
configuration.
The next two dialog boxes,
shown in Figures 3 and 4,
allow the user to set the
vessel sizes and load
conditions. The first dialog
box, “Forward Tanker Size
and Load Condition,” allows
the user to select a size
from a list of eleven generic
tankers for the first, or
Figure 3. Dialog box for selecting the forward tanker size
forward, vessel.
It also
and load condition.
allows the user to select a
loading condition, either fully loaded or ballasted, which enables the program to use the
correct current and wind load coefficient curves from the OCIMF data. The second
dialog box,”Forward Tanker Size and Hawser Data,” allows the user to review and, if
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007
STA SPM Single Point Mooring Analysis
Revision 1
Page 5
necessary, modify various parameters that are used to determine the environmental
forces acting on the vessel. The hawser length is also entered from this dialog box. If
the vessel has a bow turret, the hawser length should be set to zero. If there are two
vessels in the configuration, a second pair of nearly identical dialog boxes will be
displayed to prompt the user to input data for the second, or aft, vessel.
Figure 4. Dialog box for setting tanker size and hawser length.
The next three dialog boxes, illustrated in Figures 5, 6, and 7, allow the user to enter
the environmental data for wind, waves, current, and water depth. Figure 5 illustrates
the first dialog box, “Current Profile Option,” which allows the user select an option to
enter either a known current profile with a current velocity defined at five points
(surface, bottom, and three intermediate points) or a single current velocity at a single
depth. Based on the user’s selection, the next dialog box, “Environmental Data,” will be
either that shown in Figure 6 or that shown in Figure 7. The left side of both dialog
boxes is identical. The upper left block is used to define wind data (velocity, elevation,
and heading), the lower left block is used to define wave data (significant wave height,
significant wave period, and heading), and the right block is used to define current data.
If the current profile is known, the dialog box shown in Figure 6 is displayed. It contains
boxes for the user to enter various depths at which the current velocity is known and the
current velocities at these depths. Note that the last depth must be the water depth. If
the current profile is unknown, the dialog box shown in Figure 7 is displayed. It
contains boxes for the user to enter one depth at which the current velocity is known,
the current velocity at this depth, and the water depth. In both dialog boxes, the current
heading is entered
in the last box on
the right.
© Stewart
Technology
Associates
Figure
5. Dialog
box
STA SPM
Manual.doc
for selecting
current
profile option.
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Figure 6. Dialog box for entering environmental data for known current profile.
Figure 7. Dialog box for entering environmental data for unknown current profile.
Figure 8. Dialog box for entering aft thrust data for thrust
supplied from vessel main screws.
© Stewart Technology Associates
STA SPM Manual.doc
The last dialog box is
used to enter information
on aft thrust. If aft thrust
is supplied by the main
screws of the tanker,
then the “Aft Thrust
Data”
dialog
box
illustrated in Figure 8 is
displayed.
The user
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enters only the amount
of available thrust. The
direction of the aft thrust
is assumed to be astern.
If aft thrust is supplied by
an attendant tug boat,
then the “Aft Thrust
Figure 9. Dialog box for entering aft thrust data for thrust
Data”
dialog
box
supplied from attendant tug.
illustrated in Figure 9 is
displayed. The user enters both the amount of available thrust and the heading of the
tug boat. If there is no aft thrust, neither of these two dialog boxes will be displayed.
Once the necessary data has been entered, STA SPM will display a spreadsheet
similar to that in Figure 1 (page 2). This spreadsheet will list most of the data entered
via the above dialog boxes in the “Input Data” section of the sheet. Cells highlighted in
pale yellow can be edited by the user for future reference. Once this data has been
verified, the user selects the “Calculate Vessel and Hawser Heading” button. STA SPM
will then display the plan view of the vessels and hawsers and calculate the vessel
heading, forces on the vessel, and hawser headings and forces. Results are displayed
in the “Load Results” section of the spreadsheet.
After the results have been calculated, the user may elect to print or save the data and
results. The input data and results can be printed using the “Print Results” button or
from the File/Print option on the menu bar. Similarly, the input data and results can be
saved to a new spreadsheet using the “Save Results” button or from the File/Save
option on the menu bar.
Note that “Save Results” will not save the entire STA SPM program, but rather it copies
the values and formats of visible cells and copies the chart as a picture to a new
spreadsheet. This file cannot be used with STA SPM for any further calculations, but it
can be used with other applications as the user requires. The file is also significantly
smaller than STA SPM.
6.0
ADDITIONAL FEATURES
STA SPM contains additional features which make the program easy to use.
When STA SPM opens, it creates a customized menu bar. This menu bar is similar to
that of many windows programs. By customizing the menu bar, STA SPM reduces the
number of errors the user can make and reduces the risk of corruption of the major
algorithms. This menu bar contains the following items:
File: used to accomplish all file management and printing tasks:
 Save: save STA SPM spreadsheet under a new name for future reference,
 Print: print STA SPM summary of input data, load results, and chart, and,
 Exit: exit STA SPM.
View/Zoom: used to adjust the window zoom rate. The default value is 75%.
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007
STA SPM Single Point Mooring Analysis
Revision 1
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Change Options: used to change any of the data specified in the input stage of the
program including the following:

HOME - restarts the analysis

Change Basic Configuration – allows user to change the number of
vessels, aft thrust availability, vessel and hawser sizes, keeping
environmental conditions unchanged.

Change Size and Ballast – allows user to change just the vessels, their
dimensions and loaded conditions and hawser lengths.

Change Environment: - change the environmental conditions acting on the
vessel(s), keeping the vessel basic configuration.

Change Thrust: change the available aft thrust or its heading (if allowed with
the current configuration).
Each spreadsheet also contains three buttons below the “Load Results” section:



Calculate Vessel and Hawser Headings: used to calculate the vessel and
hawser headings (This should ALWAYS be clicked once before final results
are used).
Print Results: print STA SPM summary of input data, load results, and chart,
Save Results: save STA SPM spreadsheet under a new name for future
reference.
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007
STA SPM Single Point Mooring Analysis
7.0
Revision 1
Page 9
SOFTWARE REQUIREMENTS
STA SPM requires the Microsoft Windows environment together with Microsoft Excel
version 5.0 or later. These software packages must be pre-installed before STA SPM
can operate.
8.0
HARDWARE REQUIREMENTS
STA SPM requires a computer with Microsoft Excel running under the Microsoft
Windows environment. The output previously printed is direct from the program onto a
laser printer. Once a printer is configured (using the Windows Control Panel and the
Printers Setup option) the output can be printed on any printer. The quality will depend
on the printer capabilities. A mouse is highly recommended.
9.0
REFERENCES
1. Prediction Of Wind And Current Loads On VLCCs. Oil Companies International
Marine Forum, England 1977.
2.
Fleet Moorings, Design Manual 26.5. Department Of The Navy - Naval Facilities
Engineering Command, Alexandria, VA 1984.
3. “Broadside Current Forces On Moored Ships”, William N. Seelig, David Kriebel,
John Headland, Civil Engineering In The Oceans.
© Stewart Technology Associates
STA SPM Manual.doc
July 2, 2007